1. Technical Field
The present invention relates to a technique for reducing power consumption of an electro-optical device having memory circuits each of which is provided for a corresponding one pixel.
2. Related Art
Portable electronic apparatuses are demanded by users to be flat and lightweight. As an electro-optical element such as a liquid crystal element and an organic electroluminescent element is suitable for fulfilling such requirements, it is widely used for an electro-optical device that functions as a display device of an electronic apparatus. Since this type of electro-optical device rewrites (i.e., refreshes) the state of each pixel for every frame regardless of the content of display, it consumes a large amount of power due to operation of a driving circuit for driving each pixel and/or a controlling circuit for control thereof, thereby making it hard to reduce power consumption.
In an effort to address the above problem, a technique for turning a pixel ON or OFF in accordance with a bit stored in a built-in static-type memory circuit has been proposed, where the memory circuit stores one bit for each pixel (Refer to JP-A-8-286170). The proposed art eliminates the need for refreshing the memory circuit, which makes it unnecessary to operate the driving circuit and other related circuits when a still picture is displayed, finally attaining lower power consumption.
According to the above-identified related art described in JP-A-8-286170, partial rewriting is achieved by configuring a data line driver in an address decoder scheme. First of all, a scan driver puts each of a plurality of transistors for memory circuit selection into a conduction state. With this scan operation, all of the transistors for memory circuit selection in one line become conductive. Concurrently therewith, a data line driver applies a data voltage for display, which is of either H level or L level, to a data bit line corresponding to a target pixel to be written as selected by an address decoder; while the data line driver concurrently applies a data voltage of the inverted level to a corresponding complementary data bit line, thereby carrying out data rewriting. The data line driver is put in a high impedance state for other data bit lines and complementary bit lines corresponding to other pixels, which are not to be rewritten, so that data which has already been written in the memory is retained.
Generally speaking, as a data line holds a large parasitic capacitance, it tends to be charged at a previously-fed electric potential even when there is no data supplied from the data line; and when memory circuit selection transistors become conductive, it is difficult to maintain previously-written data, meaning that there is a strong possibility of the occurrence of data inversion (rewriting error).
In order to prevent such a writing error from occurring in the technique described in JP-A-8-286170, it is generally known to pre-charge both of the data bit line and the complimentary bit line at H level.
However, if the data bit line and the complimentary bit line are pre-charged, the output of the memory circuit and either of the data bit line and the complimentary bit line will be short-circuited, resulting in a higher power consumption because both of them are at H level although the pre-charging thereof prevents the inversion of data.
In addition to the above, further reduction in power consumption of an electro-optical device as a unit device has been much desired, as current electronic apparatuses have to meet various requirements, including extended continuous operating time, smaller battery size, enhanced functions, to name but a few.